This invention generally relates to devices used for dispensing viscous liquids such as hot melt adhesives and sealing compounds. Such devices may be referred to as fluid control valves or dispensing guns or modules. More specifically, the present invention relates to a valve for dispensing hot melt adhesives and other viscous liquids having improved features related to increasing reliability and performance while retaining the user""s ability to adjust, repair and rebuild the device.
Hot melt adhesive systems are used in industry for applications ranging from automated product assembly to carton sealing. Thermoplastic adhesive is heated in and pumped from an adhesive supply unit. The adhesive is routed through a heat-traced hose to an application device. The application device often consists of a heated manifold and one or more valves with an application nozzle or a die. The valves start and stop the flow of adhesive to the nozzle and sometimes assist in metering the flow. Air pressure is commonly used to operate the valves.
Hot melt adhesive valves typically operate at temperatures up to 425xc2x0 F. or 220xc2x0 C. Adhesive pressure in the valve can be as high as 1200 psi. Air pressure in the air cylinders can be as high as 120 psi. Conventional valves supplied by most manufacturers will cycle about 3500 cycles per minute, with a response time of approximately 7 ms opening and 10 ms closing. The service life of currently available valves range from 5 million to about 40 million cycles.
A typical dispensing device for supplying liquid, such as hot melt adhesive, is shown in FIG. 1, and generally includes a valve body having a valve that opens and closes a dispensing orifice or die. The valve is usually operated by pressurized air to dispense discrete amounts of pressurized liquid. One or more liquid seals within the device prevent leakage or the migration of liquid between the liquid and air passages of the device. Liquid dispensing devices generally related to the present invention include a liquid passage adjacent to the dispensing orifice and an air passage or chamber at an opposite end of the device. The air passage contains a piston connected to a valve stem or needle on one side and may include a spring on the other side. Under sufficient air pressure, the piston and valve stem or needle may be moved in a direction away from the seat to dispense the liquid. When air pressure on the piston is relieved, the spring will return the stem to the normally closed position against the valve seat. Air pressure may also be used to assist in closing the valve stem assembly. These devices may include either a screw to adjust the stem/piston travel or the spring pressure, or both.
Despite the wide success of devices as described above, continuing problems exist. For example, the valve stem may be over supported against sideward movement and this may lead to increased wear of the various seals used around the stem due to indeterminate side forces. Also, typical dispensing modules have included a rigidly connected or integrally formed flange on the end of the stem bearing against the return spring. This increases the possibility that a side load is exerted on the stem by the spring and, again, this may lead to increased seal wear.
Another problem associated with dispensing hot melt adhesives is the abrasive nature of contaminants that are contained in the liquid when it is delivered to the dispensing device or that are generated internally in the device. Filters are used in the prior art in an attempt to remove contaminants from the liquid but these filters are typically located in the manifold, upstream from the dispensing device. These manifold filters often contain insufficient filter area to be effective when the manifold is supplying liquid to multiple dispensing modules. Manifold filters also do not address the problems caused by char and other contaminants generated internally in the dispensing device which can damage the valve and clog the nozzle if not trapped, leading to frequent failure of the dispensing device.
Another failure mode associated with prior art hot melt dispensing devices is caused by thermal transfer and adhesive migration from the liquid valve to the air cylinder. As seen in FIG. 1, conventional dispensing modules are configured with substantial direct contact between the air cylinder body and the valve body, including in the region proximate the piston and valve stem. This facilitates undesirable transfer of leaking adhesive and destructive heat from the valve into the air cylinder seals.
Yet another deficiency associated with prior art liquid dispensing devices is the down time caused by replacement of internal valve seals. Conventional dispensing modules use liquid seals that must be replaced periodically, usually requiring complete removal and either replacement or disassembly of the entire module. This is a time consuming, labor intensive process.
It would be desirable to provide a liquid dispensing device that may be readily substituted within applications currently utilizing existing dispensing devices or modules, but having various improvements eliminating or reducing problems such as those mentioned above.
An object of the present invention is to provide a device for dispensing hot melt adhesives and other viscous liquids that can operate for extended periods with fewer failures caused by contaminants, heat, and internal wear.
Another object of the invention is to provide means internal to the dispensing device for filtering contaminants in the liquid.
A further object of the invention is provide a liquid dispensing device that mitigates the effects of heat and liquid transfer from the valve body to the air cylinder.
Yet another object of the invention is to reduce operational downtime arising from maintenance of the seals and other internal valve components.
The present invention therefore generally provides a liquid dispensing device having a valve body with a liquid passage. A valve element having a valve seat and a dispensing orifice is operatively connected with the body. A stem is mounted for movement within the body to open and close the dispensing orifice. In accordance with the invention, the stem is mounted so that it can find its own center in the fluid chamber. The stem adopts a center due to the influence of the seal and the seat. In making the seal one of the only two bearing points against the stem, the side forces on the seal are minimal, and the wear on the seal is also minimal. The liquid seal is preferably formed from Celazole PBI as this material is the strongest and highest temperature stable plastic available. Sharp edges, useful for scraping high viscosity liquid from the stem, are machined into this material. This material is very hard, and can be abrasive to most metals. Accordingly the stem is preferably made of nickel bonded tungsten carbide, one of the hardest acid resistant materials available.
The stem and piston are connected to a spring return mechanism including a return spring for maintaining the stem in a normally closed position. In a preferred embodiment, air pressure may alternatively or additionally be used to maintain the stem in a closed position. The piston is designed to translate this force into a linear-only force. The piston has two bearing points and does not depend on the stem for location. The piston uses floating piston dynamic seal design. This allows the piston to float in the cylinder to minimize friction losses that would slow the speed or response time. The piston does not benefit from or depend on the presence of a stem for radial location in the air cylinder.
Production methods favoring machining from one side of the valve block involve compromises. The air and liquid section must be separated by a cartridge (sometimes called a spool). This design can leak liquid into the air chamber. Top machining limits the variety of applicator devices that can be connected to the valve. Bottom machining limits the diameter of the air cylinder that is critical to linear force. A valve that is useful in a wide range of applications has a bolt pattern on the bottom that supports the greatest number of application attachments. The present invention is machined from both ends with a large separation notch to physically separate the two sections. This notch reduces the chance that liquid will find its way into the air chamber. This notch also reduces the heat transfer from the liquid (hot melt adhesive) section to the air section which will increase air seal life. The notch provides high visibility of the stem and piston to help in troubleshooting. This device has a bottom-mounting pattern that is consistent with the most versatile devices in the industry. Although designed as a metric device, some even inch dimensions are used to insure interchangeability with current industry devices.
Another aspect of the invention is a filter disposed as a cylinder around the stem in the liquid chamber. This filter is the final filter for removing contaminants or degraded adhesive that may make it into the area around the stem and into the critical areas in the liquid seal/stem/seat and the nozzle. The filter is appropriate in this location because liquid flow is most often proportional to the number of modules. The filter is easily changeable by the user. Different filter mesh is available to match the characteristics of the liquid (viscosity or amount of contaminates) and the output orifice size.
In accordance with another feature of the invention, the internal filter and liquid seal assemblies are easily removable from the bottom of the dispensing device for maintenance.